1. Field of the Invention
This invention relates to glasses for eyeglass lenses, and is particularly directed to such glasses having high refractive index, low dispersion and low specific gravity.
2. Description of the Prior Art
Glasses for eyeglass lenses are disclosed in Japanese Patent laid open application Nos. 9012/1977 and 11210/1977. However, all of the glasses disclosed in these applications are of high dispersion. In the glasses of the former application, as Abbe number (.nu.d) of only 41.5 can be obtained. The glasses are not of sufficiently low dispersion. Accordingly, when used lenses having a high diopter, or having high refractive power, the margins of the lenses are colored or chromatic aberration occurs. Looking through such a lens is effected. In the glasses of the latter application, the refractive index is not sufficiently high. In the glasses disclosed therein, the highest refractive index has an Abbe number of 41, which is not of sufficiently low dispersion and, similarly to the glasses disclosed in the former application, chromatic aberration occurs. Looking through the lens is effected.
U.S. Pat. Nos. 4,057,435 and 4,084,978 also disclose glasses for eyeglass lenses. However, the glasses of the former patent have high specific gravity. When a wearer puts on the spectacles having the glasses therein of the former patent, he feels discomfort due to the weight of the spectacles on his nose. The glasses of the latter patent have the disadvantage that chemical durability is poor. Long-term use of lenses made of such glasses of the latter patent causes the lens surface to have tarnish thereon, so that the lens can not be used.